This invention relates generally to control mechanisms for variable drive apparatus and, more particularly, to a speed adjustment assembly incorporating a return to neutral mechanism (RTN mechanism).
Variable drive apparatus, whether hydrostatic, toroidal, friction, or the like, will generally have a control shaft whose rotation, via manipulation of an operator control mechanism mechanically linked to the control shaft, regulates the output of the variable drive apparatus. In the description that follows, the various RTN mechanisms disclosed herein will, by way of example only, be applied to a hydraulic apparatus. It will be understood, however, that the present invention may similarly be applied to any variable drive apparatus having a rotatable control shaft.
Many variable drive apparatus have a rotatable control shaft extending out from a housing to permit adjustment of the volume and direction of hydraulic fluid flow through a hydraulic circuit, e.g. axial piston pumps, radial piston pumps, and integrated hydrostatic transmissions. In the case of an axial piston pump, a trunnion arm or shaft extending from the housing may be rotated to adjust the angular position of a swash plate. The angular position of the swash plate is known to control the volume and direction of the pump's outflow by adjusting the stroke of the pump's pistons as disclosed in U.S. Pat. No. 6,332,393, incorporated herein by reference in its entirety. Rotation of the trunnion arm or shaft in a first direction will produce a flow of hydraulic fluid in a first direction through a hydraulic circuit, while rotation of the trunnion arm or shaft in the opposite sense will produce a flow of hydraulic fluid through the hydraulic circuit in the opposite direction. In between these rotational positions there exists a neutral state where the axial piston pump is known to have zero displacement. In a hydraulic circuit where the axial piston pump is paired with a hydraulic wheel motor, e.g. in a utility vehicle, the neutral state represents a condition where the vehicle is stopped and does not creep in any particular direction. This is a necessary condition for predictable vehicle operation.
In the past, various means have been utilized for returning a hydraulic apparatus to a neutral state once an operator applied force is removed from the control shaft linkage, e.g. use of a torsion spring acting upon a control arm engaged to the trunnion arm or shaft. More recently, ‘scissors’ style RTN mechanisms have been applied to the rotatable control shafts of hydraulic pumps, integrated hydrostatic transmissions and transaxles, as disclosed in U.S. Pat. Nos. 6,715,284 and 7,134,276, which are incorporated herein by reference in their entirety. While scissors style RTN mechanisms may bring greater flexibility to the manner in which an RTN mechanism may be applied to a hydraulic apparatus, the cumulative machining tolerances of the components of a scissors style RTN mechanism may introduce a certain amount of hysteresis to the control linkage, delaying vehicle responsiveness or permitting vehicle creep. This can occur because combinations of machining tolerances at opposing limits may result in play between stops on fixed components and mating surfaces on rotating components of a scissors RTN mechanism. If a control arm engaged to the trunnion arm or shaft has some degree of free play before contacting a scissor return arm, a reproducible neutral setting will be difficult to achieve and maintain.